South Africa coach Mickey Arthur has weighed into the debate surrounding the Umpire Decision Review System (UDRS) by saying he views its use with "mixed feelings".

He has also questioned the accuracy of the predictive element of Hawkeye technology which has been used to assess leg before wicket decisions.

"I was always in favour of the UDRS but now that we have seen the system in operation for a decent period of time, I have mixed feelings," said Arthur, writing in the February edition of the Wisden Cricketer magazine.

The UDRS system was responsible for a flashpoint in last week's fourth and final Test between South Africa and England in Johannesburg.

Australian television umpire Daryl Harper failed to overturn a not out decision against South African captain Graeme Smith because he allegedly did not turn up the sound on an audio feed from the stump microphone.

Smith, who was on 15 at the time, went on to make a century and the Proteas won by an innings at the Wanderers to square the series at 1-1.

England were so infuriated they lodged an official complaint with the International Cricket Council (ICC), who have opened an ongoing investigation.

Arthur said technology did "more good than bad" but added he had "some misgivings", particularly the way in which slightly different systems were in operation around the world.

"We have to standardise the use of the UDRS across the world by using all the tools available. If 'Hot Spot' and 'Snicko' are used in one series but not another then the system is half-baked."

Replay technology in other sports, such as the ball hitting the line in tennis or being grounded for a try in rugby union, is used to determine matters of fact.

But the final question any umpire must answer when ruling on an lbw decision - would the ball have gone on to hit the stumps? - is a matter of opinion.

And Arthur is not convinced Hawkeye's judgement should be substituted for that of an umpire.

"I'm not 100 percent convinced about the predictive element of Hawkeye and I don't think many players are either."

In order for UDRS to be activated, players must appeal against the on-field umpire's decision and there were times when both sides appeared to be stretching the rules in this regard to breaking point.

"The other issue that needs to be clarified is the amount of time taken to decide whether to call for a review," Arthur said.

"I think it's fair to say that both South Africa and England did take longer than we should have done at times."

But Arthur said that, for all the controversy, UDRS was succeeding in its core task of eliminating the rank bad, as opposed to arguable, decision from international cricket.

"If the UDRS can eliminate the absolute shocker, then it is doing its job," he said.